Mesoporous slit-structured NiO materials were prepared through a simple hydrothermal route with sodium dodecyl benzene sulfonate (SDBS) as an additive. The as-prepared NiO samples presented high specific capacitance of over 1700 F g⁻¹ in the potential range from 0.10 to 0.56 V (vs. Hg/HgO/6 mol L⁻¹ KOH) at a constant current of 2 A g⁻¹, and good capacitance retention of 90% after 1000 continuous charge–discharge cycles. Only the NiO electrode materials with uniform slit-structured mesopores, which were confirmed through nitrogen adsorption–desorption isotherms and high-resolution transmission electron microscope, delivered excellent capacitances far beyond any previous report up to now. Pore structures (including pore shape, size, and distribution) are dominant factors in pseudocapacitor materials.